1. Field of the Invention
The present invention relates generally to optical storage systems, and more particularly to a gain calibration method for an optical storage servo system.
2. Description of Related Art
An optical storage servo system is used to control the positioning of an optical lens, so that a track on an optical disk will be positioned accurately beneath the lens. In contrast to a hard disk drive, an optical storage system uses removable media. As a result, both a loader for the disk, and the disk itself, may introduce disturbances during operation of the optical storage system. An optical storage servo system may have several feedback control loops, such as a focus loop and a radial loop, to achieve high performance and robustness against OPU (optical pickup unit) actuator variations and other mechanical disturbances coming from disks or an actuating mechanism.
FIG. 1 illustrates an example of a simplified servo feedback loop for an optical storage system. As shown, the loop may have a dynamic plant P and a compensator C. The plant P may have an actuator P1 and a sensor P2. As a result:P=P1·P2  (1)
The actuator P1 may include a digital/analog converter (D/A), a motor driver circuit, and a VCM (Voice Coil Motor) in an OPU/loader. Since optical storage systems use low cost motors, each loader may have a different actuator gain K2 and may introduce disturbances or uncertainty. The sensor P2 may include an optical photo diode, and a servo signal generator module in firmware. The sensor P2 may detect an error signal between the OPU's actual position and a target position, and may have different gains K1 and introduce plant uncertainties. The sensor gain K1 may include a focus sensor gain and a radial sensor gain. The compensator (or controller) C may receive the output of the sensor, as a servo signal, and generate a control effort to drive the actuator and suppress the disturbances or uncertainty. The compensator C may be a control HR (Infinite Impulse Response) filter implemented in firmware. The open loop transfer function of the servo feedback loop shown in FIG. 1 is as follows:L=C·K1·K2·P1·P2  (2)where K1 represents a sensor gain, K2 represents an actuator gain, C represents a compensator, P1 represents the actuator, and P2 represents the sensor.
The gain calibration for an optical storage servo system may include calibration of the actuator gain K2 and the sensor gain K1. Since there are big gain variations in inexpensive optical storage systems, it is very difficult to accurately measure the actuator gain K2 in open loop settings. Currently available technologies use Loop Gain Calibration (LGC) in closed loop operation to calibrate the actuator gain K2 by measuring either the magnitude or the phase of the transfer function (2), as shown in FIGS. 2A and 2B.
The curve for magnitude measurement is shown in FIG. 2A. A fixed frequency sine wave may be injected into the servo feedback loop shown in FIG. 1 at point A as a reference signal. The frequency of the sine wave may be the zero dB cross-over frequency determined by the compensator C. The magnitude of the transfer function (2) may be measured at point B. Since the target is zero dB or 1, the actuator gain K2 is the inverse of the loop magnitude measurement at the cross-over frequency.
The curve for phase measurement is shown in FIG. 2B. A fixed frequency, fixed magnitude sine wave may be injected into the servo feedback loop shown in FIG. 1 at point A as a reference signal, and the phase difference between the servo error signal at point B and the injected sine wave may be obtained. The sine wave may be, for example, 1.36 kHz. The phase difference may be compared with a target. A trial-and-error method may be used to make the phase difference to approach the target, so as to search for the actuator gain K2. One problem of the phase detection based LGC is that it suffers from defective disks, since it is difficult to implement a defect protection scheme when using the try-and-error method.
Currently available technologies use an open loop sensor peak-to-peak measurement to calibrate the sensor gain K1. In focus sensor gain calibration, an open loop focus ramp may be performed so that the peak-to-peak value of a focus error S-curve signal may be measured and compared with a target value. The S-curve peak-to-peak value is shown in FIG. 3 as the “pFocusError.” Similarly, the radial sensor gain may be calibrated by measuring the peak-to-peak value of a radial error signal in a radial open loop. One problem is that the method can only be used with single layer disk servo systems, e.g., CDs or single layer DVDs. However, multilayer format optical systems, e.g., red laser DVDs, blue laser BDs and HD-DVDs, are becoming popular, and the focus sensor gain K1 at inner layers may have a different S-curve than that of the outermost layer. Accordingly, the sensor gain K1 cannot be accurately calibrated at inner layers with the currently available technology.
In addition, the currently available technologies assume that the compensator C is fixed, and therefore include the compensator C in the path from point A to B and calibrate the loop gain of the optical servo system. The currently available technologies also assume that the variation of the actuator gain K2 is the same as that of the transfer function (2) since the compensator C is constant and the sensor gain K1 can be calibrated very accurately by the open loop sensor peak-to-peak measurement. However, in some optical storage servo systems, the spindle speed may depend on disk conditions, and a particular compensator C may be provided for each spindle speed. When the frequency of the injected sine wave stays the same, if C changes, the zero crossing point may move and may affect the calibration of the actuator gain K2.
Therefore, it may be desirable to provide a method to improve the calibration of the optical storage servo system.